A variety of solar batteries (solar battery modules and solar battery panels) comprising external lenses for yielding large output power by means of a small light receiving area have been proposed. However, because the larger areas are realized in silicon solar batteries and production cost of solar battery cells and solar battery modules is reduced, light collection by external lenses is less used.
On the other hand, in the solar battery using expensive compound semiconductors such as gallium arsenide (GaAs), light collection by external lenses is assumed to be economical and proposed in many documents.
The U.S. Pat. No. 4,136,436 and the U.S. Pat. No. 6,204,545 by the inventor of the present application propose a spherical or partially spherical solar battery cell made of granular silicon crystal as a technique for efficient use of expensive silicon raw material.
The inventor of the present application proposed in the Japanese Laid-Open Patent Publication No. 2001-168369 a solar battery module having spherical solar battery cells in which a reflecting plate is provided on the back in a close contact manner. The inventor also proposed in the International Publication No. WO03/056633 a spherical solar battery cell housed in a synthetic resin capsule having a diameter larger than the cell and filled with a synthetic resin for light collection. They have a smaller collecting power compared with use of external lenses, however they can be realized in a relatively simple structure.
The U.S. Pat. No. 5,482,568 discloses a micromirror solar battery in which multiple cone-shaped reflecting mirrors are provided in a case, a solar battery cell having a flat light-receiving surface is placed at the bottom of each cone, the sunlight collected by the cone illuminates the top surface of the solar battery cell, and the heat is released from the underside of the cone. The flat solar battery cell receives light only at the top surface and the reflection loss is not small. Therefore, it is difficult to sufficiently increase the incident light usage rate. Furthermore, this micromirror solar battery has the solar battery cells at the bottom of the case so as to prevent the solar battery cells from heating up due to light collection.
The U.S. Pat. No. 5,355,873 discloses a light collection type solar battery module having spherical solar battery cells. A thin metal sheet (common electrode) has multiple nearly semispherical recesses with reflecting inner surfaces. Legs are formed at the centers of the recesses for supporting solar battery cells. A conductive mesh supports multiple solar battery cells at their middle parts. The multiple solar battery cells are set in multiple recesses and electrically connected to the legs. The multiple solar battery cells are connected in parallel by the conductive mesh and sheet. The solar battery cells have no electrode at the top, bottom, or either end and, therefore, the electric current distribution is uneven within a solar battery cell. Hence, it is difficult to improve the electric power generation efficiency. Furthermore, all solar battery cells mounted on the sheet are connected in parallel, which is inconvenient for increasing the output voltage of the solar battery module.
The US Laid-Open Patent Publication No. 2002/0096206 discloses a solar battery module in which spherical solar battery cells are provided in the centers of multiple partially spherical recesses, respectively, the recesses each have a reflecting inner surface, multiple recesses are formed by two thin metal plates and an insulating layer between them, and the two thin metal plates are connected to the positive and negative electrodes of the spherical solar battery cell at the bottom part thereof to connect in parallel multiple solar battery cells.
In the above solar battery module, the spherical solar battery cells are electrically connected to the two thin metal plates at the bottom part. This causes a drawback that the distance between the upper half light receiving surface and the positive and negative electrodes of a spherical solar battery cell is large and the resistance loss upon output electric current retrieval is increased. Furthermore, all solar battery cells of the solar battery module are connected in parallel, which is inconvenient for increasing the output voltage of the solar battery module.
The inventor of the present application disclosed in the International Publication No. WO02/35612 a rod-shaped light receiving or emitting semiconductor element having a pair of electrodes on either end face and a solar battery module using the semiconductor element. However, when this rod-shaped semiconductor element has a higher length/diameter ratio, the resistance between the electrodes is increased. Therefore, the ratio is desirably set for approximately 1.5 or lower.
Patent Document 1: U.S. Pat. No. 4,136,436;
Patent Document 2: U.S. Pat. No. 6,204,545;
Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-168369;
Patent Document 4: International Publication No. WO03/056633;
Patent Document 5: U.S. Pat. No. 5,482,568;
Patent Document 6: U.S. Pat. No. 5,355,873; and
Patent Document 7: US Laid-Open Patent Publication No. 2002/0096206.